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 equation of available energy) must be the case with any system whose available energy is exactly proportional to the absolute temperature.

The advances which were effected in the last quarter of the nineteenth century in regard to the conduction of electricity through liquids, considerable though these advances were, may be regarded as the natural development of a theory which had long been before the world. It was otherwise with the kindred problem of the conduction of electricity through gases: for although many generations of philosophers had studied the remarkable effects which are presented by the passage of a current through a rarefied gas, it was not until recent times that a satisfactory theory of the phenomena was discovered.

Some of the electricians of the earlier part of the eighteenth century performed experiments in vacuous spaces; in particular, Hauksbee in 1705 observed a luminosity when glass is rubbed in rarefied air. But the first investigator of the continuous discharge through a rarefied gas seems to have been Watson, who, by means of an electrical machine, sent a current through an exhausted glass tube three feet long and three inches in diameter. "It was," he wrote, "a most delightful spectacle, when the room was darkened, to see the electricity in its passage; to be able to observe not, as in the open air, its brushes or pencils of rays an inch or two in length, but here the coruscations were of the whole length of the tube between the plates, that is to say, thirty-two inches." Its appearance he described as being on different occasions "of a bright silver hue," "resembling very much the most lively coruscations of the aurora borealis," and "forming a continued arch of lambent fame." His theoretical explanation was that the electricity "is seen, without any preternatural force, pushing itself on through the vacuum by its own elasticity, in order to maintain the